"... Since the
inception of IRS Criminal Investigation, our mission has been to follow
the money. And almost from the inception, the question of "Why is IRS
involved?" has been asked. The fact is, no matter what the source, all
income earned, both legal and illegal, has the potential of becoming
involved in crimes which fall within the investigative jurisdiction of
IRS CI. It is the objective of CI to utilize the broad authority of
Title 26 and 31, tax, currency reporting and forfeiture and Title 18
money laundering, conspiracy and forfeiture to identify, investigate and
prosecute the most significant tax, currency and money laundering
offenders; and to pursue the assets of those offenders both domestically
and internationally for criminal tax and asset forfeiture purposes. ..."
http://www.treas.gov/irs/ci/docfinancialcrimes.htm

When to contact the IRS-CID:

When you have
knowledge or evidence of:

Income Tax Evasion

Frivolous filers/non-filers who
challenge the constitutionality of the American tax
system

If you are located outside the U.S.,
then you can contact the nearest American Embassy or Consulate and speak
with a Legal Attach?(a-ta-shay). Go here
for a list of Legal Attach?: http://www.fbi.gov/contact/legat/legat.htm

The Attach?will either direct you to the correct office or contact the IRS
Criminal Investigation Division on your behalf.

If you are calling in a tip that you suspect someone of a
violation, before you call write down what you want to say or make some
of notes so you remember everything you want to report. There are
few things more frustrating than remembering information you wanted to
hand over after you've hung up the phone.

The IRS Criminal Investigations Division is open to calls
from non-US residents (citizen residents of foreign countries) who want
to report U.S.-based criminal activity or crimes perpetrated by US
citizens living abroad.